Radio receivers such as AM and frequency modulation (FM) receivers are well known and are pervasive. Conventionally, these receivers have been formed of analog circuitry to receive an incoming radio frequency (RF) signal, downconvert the signal, and demodulate the downconverted signal to obtain an audio signal for output. Typically, the circuitry for AM and FM receivers, even in a combined radio, includes separate dedicated paths for AM and FM operation. While such analog-based circuitry may perform well, the area associated with this analog circuitry typically exceeds that used for digital circuitry, and the analog receivers typically include many discrete components. In contrast, digital circuitry is generally available in ever-decreasing sizes, as the benefits of advanced semiconductor processes provide for greater integration. Furthermore, the cost of digital integrated circuits (ICs) is generally less than corresponding analog circuitry.
Accordingly, some radio receivers are being designed to incorporate greater amounts of digital circuitry. While such circuitry may improve performance and can be formed in small packages, typically there are complexities in processing RF signals that require significant digital processing to match the relatively simple circuitry of an analog receiver.
Radio receivers include various components including circuitry for locking the receiver onto a desired channel. Such circuitry often includes automatic frequency control (AFC) circuitry to track a carrier frequency of the desired channel of an incoming signal. Furthermore, because the radio spectrum is relatively crowded, in addition to a desired channel, one or more adjacent channels may be relatively close in frequency to the desired channel. As such, receivers typically include a channel filter which may be in the form of a bandpass filter to remove unwanted channels and pass a desired channel. However, some amount of undesired channel information can still be present in the channel filter output.
Circuitry for AFC as well as channel filtering can be located in various places in a signal processing path of a receiver. In some receivers, the channel filter may be placed in front of the AFC circuitry, while in other receivers the AFC circuitry may come before the channel filter. In either instance, undesired effects can occur. If the channel filter is placed ahead of the AFC circuitry within a signal processing path, the incoming signal may not be frequency centered within the channel filter, causing frequency response distortion. If instead the AFC circuitry is placed in front of the filter within the signal processing path, one or more non-desired channels may influence the AFC circuitry and cause noise or distortion.